1. Field of the Invention
The present invention relates to a laser beam machining method in which a wafer such as a semiconductor wafer is irradiated with a laser beam along each of planned dividing lines formed in the wafer, whereby denatured layers are formed inside the wafer along each of the planned dividing lines.
2. Description of the Related Art
In the semiconductor device chip manufacturing process, planned dividing lines arranged in a grid-like form are formed in a face-side surface of a substantially circular disk-like semiconductor wafer to demarcate a plurality of rectangular regions in the face-side surface, then electronic circuits such as ICs (Integral Circuit) and LSIs (Large Scale Integration) and/or minute electromechanical devices called MEMS (Micro Electro Mechanical System) are formed in the rectangular regions, and thereafter the wafer is cut along the planned dividing lines to obtain semiconductor chips based on the rectangular regions. The thickness of the wafer is generally about 600 to 800 μm, and the wafer may be thinned by grinding the back-side surface thereof, as required. Depending on the use of the semiconductor chips, however, the wafer in the original thickness state may directly be divided, without thinning.
As the means for cutting the wafer, there is generally used a dicing method in which a thin disk-shaped blade being rotated at high speed is caused to cut the wafer. The dicing method has several advantages, one of which is that flat and sharp cut sections can be obtained. In the dicing method, however, the width of the planned dividing lines between the chips has to be not less than a value corresponding to the thickness of the blade used (mainly, about 10 to 30 μm), so that the cutting margin is comparatively large. This is disadvantageous from the viewpoint of increasing the number of chips obtained from a single wafer as much as possible to thereby enhance productivity.
On the other hand, in recent years, a laser beam machining method has come to be adopted in which the inside of a wafer is irradiated with a penetrating laser beam along each of planned dividing lines to form a denatured layer lowered in physical strength, and then an external force is exerted on the wafer to split the wafer along the planned dividing lines, thereby obtaining individual chips. The cutting margin in the laser beam machining method is much smaller than that in the dicing method, and, therefore, the laser beam machining method is said to be advantageous from the viewpoint of productivity. However, in the case of splitting the wafer by the laser beam machining method while the wafer is kept comparatively thick without being thinned, there would be a problem that formation of one denatured layer through one-time irradiation with the laser beam for each of the planned dividing lines may make smooth dividing of the wafer impossible, due to the small proportion of the denatured layer based on the wafer thickness.
In order to solve this problem, there is known a technology in which irradiation with the laser beam is conducted in a plurality of stages to form a plural number of denatured layers for each of the planned dividing lines, thereby ensuring that the splitting of the wafer can be carried out easily and accurately (refer to Japanese Patent Laid-Open No. 2002-205180). In addition, a technology has been proposed in which the wavelength of the laser beam for irradiating the wafer therewith is changed from the conventional value of 1064 nm to a value, promising better penetration into the wafer, of 1100 to 2000 nm (preferably, 1300 to 1600 nm) so as to form the denatured layer efficiently (refer to Japanese Patent Laid-Open No. 2006-108459).
For example, in the case of splitting a wafer with a thickness of 625 μm by irradiating the wafer with a laser beam, the use of a laser beam with the conventional wavelength of 1064 nm results in that a large number of denatured layers, i.e., generally about 18 denatured layers, have to be formed in the thickness direction of the wafer. This means the need to repeat the laser beam irradiation 18 times for each of the planned dividing lines, so that the number of times of irradiation is too large to obtain good machining efficiency. On the other hand, the use of a laser beam with a wavelength of 1342 nm makes it possible to achieve the splitting by forming about eight denatured layers for each of the planned dividing lines, whereby the machining efficiency is improved. However, even by irradiation with the laser beam with a wavelength of 1342 nm, it is in practice difficult to form a multiplicity of denatured layers proximate to each other in the thickness direction of the wafer. It is said that, where a multiplicity of denatured layers are formed, a cracked layer extends from a given one of the denatured layers thus formed toward the adjacent denatured layer to reach the adjacent denatured layer, whereby smooth splitting of the wafer can be realized. Therefore, if the multiplicity of denatured layers cannot be formed closely to each other, the reaching of the cracked layer to the adjacent denatured layer cannot take place easily, so that defective splitting is liable to occur.
The reason for the difficulty in forming the adjacent denatured layers proximately to each other is inferred to reside in that strains or defect layers on a crystal level are formed in the vicinity of the denatured layer once formed and the cracked layer extending from the denatured layer, and, even when that portion is irradiated with a laser beam, good condensing of the laser beam cannot be achieved, so that sufficient multiple photon absorption cannot be attained. Accordingly, in the case of forming a second denatured layer adjacent to a formerly formed first denatured layer, it is necessary to form the second denatured layer at a position somewhat spaced from the cracked layer extending from the first denatured layer, with the result that the cracked layer extending from the first denatured layer does not reach the second denatured layer, and defective splitting is liable to occur.